fix(ns): class_exists() must use FQCN after PSR-4

Bare class_exists('Balance') looks up global Balance, always false
under AKSRT\* namespaces → REST 403 on balance exchange and friends.
Rewrite string checks to AKSRT\{Domain}\{Class}.

src/AKSRT/Shop/CartService.php

@@ -159,7 +159,7 @@ class CartService {
             $guest = get_post_meta($product_id, '_aksrt_guest_buy', true);
             if ($guest === '0' || $guest === '') {
                 // Fall back to inheritance chain via TermMeta
-                if (class_exists('TermMeta')) {
+                if (class_exists('AKSRT\Shop\TermMeta')) {
                     $resolved = TermMeta::resolve_inheritance($product_id, 'guest_buy');
                     if (empty($resolved)) {
                         return 'login_required';